Given the high abundance derived for the radical CH\(_2\) CCH [1]}, reactions between radicals,
such as CH\(_2\) CCH + C\(_2\) H\(_5\)  and C\(_2\) H\(_3\)  + CH\(_2\) CHCH\(_2\) , are also potential sources of
\(c\) -C\(_5\) H\(_6\) .
Routes involving ions, such as condensation rections between cationic and neutral
hydrocarbons [2]}, may also lead to \(c\) -C\(_5\) H\(_6\) .
Theoretical studies on these reactions would help to evaluate their role in
the synthesis of cyclopentadiene. Whatever the formation route to \(c\) -C\(_5\) H\(_6\)  is,
this hydrocarbon
cycle is the most obvious
precursor of the two cyanide derivatives \(c\) -C\(_5\) H\(_5\) CN detected in TMC-1 [3]}, [4]}.
